1. Field of the Invention
The present invention relates generally to marine communication, and more particularly to marine communication utilizing a remotely controlled radio telemetry buoy capable of serving as a long-range communication link between an underwater acoustic interrogation/receiving transponder system and a remotely located surface platform.
2. Description of the Prior Art
It is known in the art to deploy portable sonobuoys for temporary in-water tracking. Typically, a hydrophone is mounted below the buoy and receives acoustic tracking signals. After pre-amplification, the composite in-water signal is used to ultimately modulate a very small (0.5-1 watt) FM transmitter. The telemetry signal is relayed back to shore or to an aircraft or ship. Because the buoy antenna height is commonly only a few feet, transmission from the buoy does not extend over the horizon and is thus limited to line-of-sight, or typically 8-10 miles at VHF operating frequencies between 30 and 300 MHz.
Remote control systems for such buoys have historically been implemented using a standard U.S. Navy control frequency. At this frequency, an AM carrier with a primitive tone modulation scheme has been used. However, this modulation scheme and its decoder are severely limited in that only a few, i.e., typically three, independent commands are possible.
Another modulation scheme uses a DTMF (dual tone, multiple frequency) encoder to modulate an AM transmitter at UHF. This allows the use of inexpensive "touch tone" telephone technology. Thus, using a keypad encoder and decoder circuit board, sixteen unique decoded outputs can be obtained by pressing only one keypad button per command. Combinations of two and three keypad digits may be used to further expand the command set, with increased complexity at the decoder end.
However, both modulation schemes have propagation limitations owing to the UHF control frequency which is only useful at line-of-sight ranges unless transmit power of 50-100 watts and impractical shipboard antenna heights (75-100 feet) are used. Moreover, even if these conditions are met, control ranges of greater than 12-15 miles are not consistent and reliable.
UHF operating frequency radio modems can be used for remote control of the buoy, but in general these are manufactured for the 400-500 MHz frequency range and the power is limited to only a few watts. These frequencies are also limited to line-of-sight and propagation is quite sensitive to small objects such as passing boats.
The above noted deficiencies in the art of sonobuoy telemetry and control are increasingly important in view of the U.S. Navy's development of the Portable Tracking System (PTS). The base component of the PTS is an underwater acoustic transponder system. The transponder system includes a plurality of bottom deployed transponders that are acoustically interrogated by a pinger. In an effort to improve precision with respect to transponder interrogation and range tracking, each transponder communicates via phase coded, digital control/data words. Furthermore, it is desired to increase PTS telemetry uplink and control range beyond the current state-of-the-art line-of-sight distance limitation.